Various devices have been under active study and development, particularly those based on electroluminescence (EL) from organic materials. The use of phosphorescent materials has been a major breakthrough in boosting electroluminescence efficiency since they allow simultaneous harvesting of both singlet and triplet excitons.
Unfortunately, the emission lifetimes of these phosphorescent complexes are relatively long, leading to undesired triplet-triplet annihilation during the operation of a device. To overcome this problem, phosphorescent emitters are doped into organic host materials.
Selecting a suitable host material for the phosphorescent dopants remains one of the critical issues in phosphorescence-based organic light emitting diodes (OLED). An ideal host material would meet the following intrinsic requirements: a triplet energy gap (Et) larger than that of the triplet dopant to prevent reverse energy transfer from the guest back to the host, good carrier transporting properties to balance the charge flux and reduce the driving voltage, thermal and morphological stability to extend the device operational lifetime.
Well-known host materials for guest-host systems include hole-transporting 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) and electron-transporting aluminium 8-hydroxyquinoline (Alq3), which have been used in OLED. Those host materials have suitable properties for green and red emitters.
In contrast, highly efficient blue-light emitting phosphorescent devices remain rare, mainly because of the lack of suitable host materials possessing both charge transporting characteristics and high triplet energy.
Several host materials for better phosphorescent emission have been reported. Due to their charge conducting ability, photophysical and redox properties, sufficiently large triplet energies and carrier-transport properties, carbazole-based compounds have been actively studied. Carbazole-based materials simultaneously possess sufficiently large triplet energies and carrier-transport properties.
For carbazole-based molecules to acquire enough morphological stability when they are deposited as thin films, extension of molecular dimensions beyond single carbazole units to obtain sterically bulky molecular configurations is necessary. Some papers, e.g., Adv. Mater. 2007, 19, 862-866 and Synth. Mater. 2007, 157, 529-533 describe that linking carbazole units together to form polycarbazoles or adding substituents on triscarbazole compound can affect the uniformity and stability of an evaporated film.
For example, U.S. Patent Application Publication No. US 2003/205696 discloses guest-host emissive systems suitable for use in OLED in which the host material comprises a compound having a carbazole core with an electron-donating species bonded to nitrogen, aromatic amine groups or carbazole groups bonded to one or more of the carbon atoms, a large band gap potential, and high-energy triplet excited states. Such materials permit short-wavelength phosphorescent emission by an associated guest material, and the combination of said materials with emissive phosphorescent organometallic compounds such as iridium complexes is useful in the fabrication of OLED.
Japan Patent Application Publication No. JP 2009/021335 and International Application No. WO 2009/060757. and WO 2009/060780, and U.S. Patent Application Publication No. US2009/0218938 discloses OLED comprising phenyl triscarbazoles and Ir complexes.
However, none of the above-disclosed materials meets all the requirements necessary for OLED application, particularly suitable energy level, charge transport ability, processability from a solution with uniform film formation, ability to form an amorphous phase, as well as long lifetime under operational conditions of the device. Thus, there has been a need to develop new host materials, which are capable of satisfying all of the requirements indicated above.